In the art of recovering oil from subterranean reservoirs it is known that substantial volumes of oil are left in the reservoir after all effective primary production techniques have been utilized. In order to recover additional oil from reservoirs which have been depleted by primary production techniques, it is necessary to resort to secondary recovery techniques, which generally involve driving the oil from the reservoir with one or more fluids. For example, the continuous injection of water, gases such as natural gas, air, carbon dioxide, etc. or, in order to reduce the volume of expensive driving fluids, introducing a slug of a fluid such as propane, a surfactant solution, etc. followed by a drive fluid such as water, natural gas or air. Certain of these techniques may be carried out under conditions such that at least the fluid in contact with the reservoir oil is miscible with the oil and preferably, in the case of a slug-type process, the slug is also miscible with the ultimate driving fluid. Miscible displacement is advantageous to the extent that, at least theoretically, 100 percent of the oil is displaced by the miscible fluid in the area contacted by the miscible fluid, which displacement efficiency is substantially above that obtained when a fluid immiscible with the oil is used. However, the maintence of miscibility throughout the drive from injection wells to production wells is difficult at best and in some cases impossible. Another problem in both miscible and immiscible drives which utilize gas as either the sole driving fluid or the ultimate driving fluid in a slug-type process is that unfavorable mobility ratios exist in the reservoir. Specifically, the more highly mobile gas tends to channel through the reservoir thereby sweeping only a very limited area of the reservoir and/or breaking through a slug material and thereby partially or completely eliminating the advantages of the high displacement efficiency of the slug. Of the above mentioned techniques the use of a slug of a surfactant solution has a number of advantages. First of all the surfactant reduces the interfacial tension in the reservoir, thus substantially improving displacement of the oil. Secondly, since the slug of surfactant solution is a liquid and can be driven by water, the disadvantages of unfavorable mobility ratios are eliminated or substantially reduced. Finally, surfactant recovery techniques can be utilized in reservoirs which have already been subjected to secondary recovery techniques, particularly where the reservoir has been produced to its economic limits by water flooding. In the last instance the surfactant technique is referred to as a tertiary oil recovery technique.
A wide variety of surfactant oil recovery techniques have heretofore been proposed. For example, the surfactants may be used in both systems forming microemulsions and in systems which do not form microemulsions. The surfactant system may also be utilized under conditions such that displacement of the oil is under either miscible or immiscible conditions. As indicated previously, miscible displacement in general is at least theoretically highly desirable but has the inherent difficulties of maintaining miscible displacement throughout the reservoir and a miscible surfactant drive presents problems of maintaining interfacial tension sufficiently low to provide immiscible displacement after miscibility has been lost. One technique which has received increasing attention in recent years is the utilization of a microemulsion under immiscible conditions. This technique can be practiced either by forming a multiphase microemulsion system comprising a water phase, a microemulsion phase, and an oil phase at the surface of the earth by mixing oil, brine and surfactant and thereafter injecting at least the immiscible microemulsion phase into the reservoir. However, this technique requires substantial amounts of oil, thereby increasing operating costs, and to the extent an oil other than the reservoir oil is utilized, problems occur due to differing phase behaviors of different oils. It has also been proposed to prepare a surfactant system including water, an electrolyte, at least one surfactant and optionally a cosurfactant at the surface of the earth, inject the surfactant system into the reservoir and thereby form the immiscible microemulsion in situ in the reservoir. It is preferable in such cases to prepare a surfactant system capable of forming the previously mentioned multiphase system of water, microemulsion and oil. It is also preferred, in practicing this technique, to employ synthetic or petroleum hydrocarbon sulfonates as a surfactant due to their availability and for economic reasons. One of the most important factors in the utilization of hydrocarbon sulfonates in surfactant systems designed to form an immiscible, microemulsion in situ in a reservoir is a knowledge of the average equivalent weight of the sulfonate. While the average equivalent weight of a number of commercially available sulfonates have been established, this is not the case with all commercially available materials and, in addition, it is often necessary to tailor the sulfonate for use in a particular oil recovery process, since the particular sulfonate which would be most effective may not be commercially available.
Obviously, knowledge of the average equivalent weight of hydrocarbon sulfonates is also highly desirable where such sulfonates are to be utilized for other purposes, such as detergents, etc.
Characterization of petroleum sulfonates by the average equivalent weight method described in ASTM Procedure D-855-56 is usable only for sodium sulfonates and is a time consuming procedure. Further, analytical characterization methods, based on anionic surfactant dye complexes, are also subject to various problems due to impurities in the dye, salt effects and the interference of unreacted oil in the sulfonate.
In light of the above it is highly desirable to provide a fast reproducible and accurate technique for determining the average equivalent weight of hydrocarbon sulfonates and to be able to utilize knowledge of the average equivalent weight of such sulfonates in the recovery of oil from subterranean reservoirs.
It is therefore an object of the present invention to provide a fast accurate and reproducible technique for characterizing hydrocarbon sulfonates.
Another object of the present invention is to provide a fast accurate and reproducible technique for characterizing hydrocarbon sulfonates for use in the displacement of oil from a subterranean reservoir.
A further object of the present invention is to provide a fast accurate and reproducible technique for determining the average equivalent weights of hydrocarbon sulfonates of unknown average equivalent weight.
Yet another object of the present invention is to provide an effective fast and accurate technique for determining the average equivalent weight of hydrocarbon sulfonates which permit one to produce sulfonates tailored to be most effective in oil recovery processes.
Another and further object of the present invention is to provide an improved technique for the preparation of a surfactant system including at least one hydrocarbon sulfonate for the displacement of oil from a subterranean reservoir.
A still further object of the present invention is to provide an improved technique for the displacement of oil from a subterranean reservoir by the injection of a surfactant system containing at least one hydrocarbon sulfonate.
These and other objects and advantages of the present invention will be apparent from the following description.